fixes for ofDrawGridPlane, ofDrawGrid

+ fixes issue where central axis would not be drawn when non-integer tick count specified
+ fixes ofDrawGrid API to specify stepSize and numberOfSteps rather than scale, tickNumber
+ ensures numberOfSteps be a positive integer
changelog update
Merge pull request #3316 from tgfrerer/fix-ticks+drawGridPanes

fixes for ofDrawGridPlane, ofDrawGrid